Implement support for environment variable-based reporters

[FWest98]

Description

This PR implements support for configuring reporters through environment variables, similar to how this is supported in the C++ version of this library: https://github.com/approvals/ApprovalTests.cpp/blob/master/doc/how_tos/SelectReporterWithEnvironmentVariable.md

The solution

As implementation, I added an additional reporter EnvironmentVariableReporter, that is included by default in the FrontloadedReporter. When no environment variable is specified, this will not change the previous behaviour. When the environment variable is specified, the selected reporter(s) will be used, and will take priority over any other configured reporters - similar to the C++ version.

This particular approach supports specifying multiple reporters separated by a comma. They are then embedded in a MultiReporter.

[LarsEckart]

hello Floris,

thank you for this contribution. I pinged @isidore - we usually invite people to our weekly pair programming session (Mondays, ~18 o'clock Amsterdam time) when we accept contributions. This gives us a chance to hear more about it or provide feedback that we then can immediately implement.

Stay tuned :)

[FWest98]

Hi Lars!

Is there any update on this?

[LarsEckart]

hey, today is the first time we get together again afetr vacations. We accept this PR but will make a small adjustment and will release it later today. Thank you for this contribution, much appreciated! :)